Space Debris Basics: What is Orbital Debris?

Orbital debris generally refers to material that is on orbit as the result of space missions, but is no longer serving any function. There are many sources of debris. One source is discarded hardware. For example, many launch vehicle upper stages have been left on orbit after they are spent. Many satellites are also abandoned at the end of useful life. Another source of debris is spacecraft and mission operations, such as deployments and separations. These have typically involved the release of items such as separation bolts, lens caps, momentum flywheels, nuclear reactor cores, clamp bands, auxiliary motors, launch vehicle fairings, and adapter shrouds.

Hubble Telescope Damage. Photograph courtesy of NASA.

Material degradation due to atomic oxygen, solar heating, and solar radiation has resulted in the production of particulates such as paint flakes and bits of multilayer insulation. Solid rocket motors used to boost satellite orbits have produced various debris items, including motor casings, aluminum oxide exhaust particles, nozzle slag, motor-liner residuals, solid-fuel fragments, and exhaust cone bits resulting from erosion during the burn.

A major contributor to the orbital debris background has been object breakup. As of August 2007 (the most recent time for which the data has been compiled), there have been 194 breakups and 51 events in which debris has been shed from an object. Since then, many more are believed to have occurred. Breakups can be caused by explosions or collisions with other objects, but the majority of breakups have been caused by explosions. Explosions can be caused by residual propellant, batteries that overheat, or in some cases, deliberate destruction of the satellite. Explosions can also be indirectly triggered by collisions with small debris.

Two events in recent years have greatly increased the amount of debris on orbit. On Feb. 10, 2009, the active Iridium 33 satellite collided with the defunct Cosmos 2251 satellite, and created about 2,000 tracked objects. On Jan. 11, 2007, the Chinese deliberately destroyed the FY-1C satellite in a test of an antisatellite weapon, creating more than 3,000 tracked objects. The tracked objects represent a small fraction of the debris objects created.

Several other collisions are known or suspected to have occurred since the beginning of the space age. In addition, the debris research community has concluded that at least one additional breakup was caused by collision. The cause of approximately 22 percent of observed breakups is unknown.

Approximately 70,000 objects estimated to be 2 cm in size have been observed in the 850-1,000-km altitude band. NASA has hypothesized that these objects are frozen bits of nuclear reactor coolant that are leaking from a number of Russian RORSATs.

At altitudes of 2,000 km and lower, it is generally accepted that the debris population dominates the natural meteoroid population for object sizes 1 mm and larger.